Many well completions include a means of injecting chemicals into the wellbore at a point in the completion for the purposes of corrosion reduction, scale reduction, hydrate reduction, well stimulation, a variety of optimisation strategies or the like. A typical installation will include a chemical injection line which is run from a surface chemical injection pump system, alongside a production tubing to terminate at a downhole location to permit a chemical injected from surface to be dosed into the production tubing, formation or other desired location.
The fluid within the injection line will be subject to hydrostatic pressure, which can often be significant in deeper wells. If this hydrostatic pressure should exceed the pressure within the production tubing (which may occur in a depleting wellbore, a wellbore subject to artificial lift or the like), in addition to any other associated resistance to the injection fluid, then the result can be the undesirable flow or cascading of injection fluid into the production tubing. This effect may be termed “hydrostatic fail-through”. If unchecked such hydrostatic fall-through will occur until the hydrostatic pressure within the injection line is in equilibrium with the production tubing pressure and other flow resistance. If the injection fluid is not continuously replenished during such cascading flow, which may be the case when injection pumps are inoperative, then the result will be the creation of a vacuum in the upper region of the injection line. Such a vacuum may present the injection line to adverse mechanical forces and stresses, such as radial collapse forces. Furthermore, the established vacuum may be defined by a pressure which is lower than the vapour pressure of the injection fluid, thus causing the injection fluid to boil. This may be compounded by the effect of the increased temperatures associated with wellbore environments.
The consequence of vacuum occurrence in chemical injection lines is that the original fluid may not be able to retain its intended state and the fluid carrier will boil off. This has the potential of many adverse effects, such as solid depositing, viscosity change, crystal formation, waxing, partial or full solidification and the like.
In order to address the problems defined above it is known in the art to utilise an appropriate injection valve near the point of injection into the production tubing, wherein the valve seeks to maintain a positive pressure within the full height of the injection line. A known injection valve includes a housing with a valve assembly which adjustably permits flow from an inlet to an outlet. Flow is initiated when the inlet pressure exceeds a threshold, and during flow the valve defines a flow restriction which establishes a back pressure on the inlet side and hence within the injection line.
A portion of a known injection valve 1 is shown in FIG. 1. A housing 2 defines an inlet 3 and an outlet 4 and a valve assembly 5 is positioned therebetween. The valve assembly includes a ball 6 which is arranged to cooperate with a seat 7, wherein flow is prevented when the ball 6 is engaged with the seat 7 and permitted when disengaged. When the ball 6 is closed against the seat 7 inlet pressure will act over the defined ball/seat sealing area 8 thus applying a force in a direction to lift the ball 6 from the seat 7, whereas outlet pressure will act over the sealing area 8 on an opposite side thus applying a force in a direction to engage the ball 6 against the seat 7. The known injection valve 1 includes a spring member 9 which applies a bias force against the ball 6 in a direction to close the ball 6 against the seat 7. Appropriate selection of the spring force may permit an appropriate resistance backpressure rating of the device 1 to be achieved.
The net force applied on the ball may be expressed by:FN=FIP−(FOP+FS)
wherein: FN=net force                FIP=force generated over seal area 24 by inlet pressure        FOP=force generated over seal area 24 by outlet pressure        FS=spring force        
At all times the force generated by the inlet pressure FIP will be acting to disengage the ball 6 from the seat 7. Accordingly, for flow to occur the net force FN must be positive in that the value of the force generated by inlet pressure FIP must be greater than the sum of the force generated by outlet pressure and the spring (FOP+FS). During flow the ball 6 will continuously adjust to seek force equilibrium, thus functioning to modify the restriction to flow between the ball 6 and seat 7 and regulate flow which inherently permits a positive pressure to be maintained within the injection line. This positive pressure will be a function of the spring force FS.
When the sum of outlet pressure force and spring force (FOP+FS) exceeds the inlet pressure force FIP the ball 6 will be moved in a direction to engage the seat 7, with the expectation that a seal will occur upon engagement. However, at the instant when the combined forces generated by the outlet pressure and spring (FOP+FS) exceed the force generated by the inlet pressure FIP, the force differential or net force FN acting to close and maintain the ball 6 in sealing engagement with the seat 7 will be extremely small. Such a low force may be insufficient to prevent leakage, particularly where the surfaces of the ball 6 and seat 7 have become contaminated. Such leakage may result in reducing pressure within the injection line and possible creation of a vacuum, which is to be avoided.
Furthermore, as fluid pressure acts over the sealing area 8 to facilitate movement of the ball 6, then in order to generate sufficient forces the area 8 must be relatively large. The provision of such a large area may present problems, for example by making it difficult to create a seal over such a large area when required. To accommodate sealing very precision components must be utilised which may be expensive. Furthermore, a larger sealing area will result in a potentially larger flow area when the ball 6 is lifted from the seat 7, which may lead to the sensitivity issues in that a very large range of flow rates will occur over only a very minute range of movement of the ball 6 relative to the seat 7.
Also, a larger sealing area may become more susceptible to contamination.
Pressure sensitive equipment, such as the known injection valve 10 described above, is typically installed with a degree of protection which isolates sensitive components of the equipment from wellbore pressure and conditions until installation is completed. Such protection may include burst disks or rupture cartridges. However, when such protective components are eventually ruptured by elevated pressures a flow surge can be created which may cause the ball 6 to be aggressively lifted from and re-engaged with the seat 7 resulting in possible damage. The ball 6 and/or seat 7 are typically formed from brittle materials, such as sapphire, ruby, ceramics, carbides, hard metals or the like which may shatter during such a surge event.